Electrostatic origin of the cooperative effect on the CO bond lengths and the amide I vibrational frequencies of the N-methylacetamide oligomers

The electric fields operating among the molecules in the oligomers of N-methylacetamide are analyzed to see the reasons for the cooperative effect on the CO bond lengths and the diagonal force constants in the vibrational subspace of the amide I mode. It is suggested that the following three factors are important for the enhancement of the electric field that gives rise to the cooperative effect: (1) the changes in the intermolecular distances (arising from the changes in the hydrogen-bond lengths); (2) the existence of the electric field directly operating between distant molecules, and (3) the polarization of the intervening molecule(s) between distant molecules. It is shown that the third factor is reasonably explained by the dipole-induced dipole mechanism. Some requirements for the correct representation of the electrostatic effects on the structures and the vibrational modes of polypeptides and proteins are proposed on the basis of this result.